1. Technical Field
The present disclosure generally relates to cooling systems for electronic equipment. More particularly, the present disclosure relates to systems and methods for cooling inverters of variable frequency drives in cooling systems for electronic equipment.
2. Background of Related Art
Compressors are deployed for many air conditioning and refrigeration systems including for commercial and industrial cooling for human comfort and refrigeration applications including systems for cooling data centers. Compressors using modern-day inverters (e.g., motor speed controllers or variable speed drives) require very tight temperature and humidity control. Inverters have been adopted on many compressor systems in order to gain greater efficiencies. As with any electrical motor controller, the inverter generates a certain amount of heat. This heat output varies based on the efficiency of the inverter and the compressor motor and the amount of load on the motor.
To ensure safe operation of the inverter, the temperature and humidity of the environment surrounding the inverter must be carefully measured and controlled. For this reason, many chiller manufacturers provide their own internal cooling systems to protect their compressor motor inverter. By doing this, they do not rely strictly on cool environments being maintained within a chiller or mechanical equipment room.
Many such inverter cooling systems are fed from systems that are served by the chiller or compressor. For example, a cooling system for a particular chiller inverter may be fed from the condenser water that is part of the chiller system. As another example, other equipment manufacturers may cool their inverters with the refrigerant that is part of the base system cycle within the chiller. Each of these examples requires condenser water or refrigerant to be pumped through the inverter's heat exchanger (extractor) to keep the inverter cool.
Various manufacturers of compressors and chillers have experienced many problems with maintaining tight control of this cooling process because of the variables associated with the cooling media, as well as the environmental variables or conditions of the space in which the inverter operates. The variables of the cooling media include the range of temperature of the cooling media. The temperature and other variables of the condenser water and chiller refrigerants vary greatly throughout the various weather and operating conditions.
The environmental variables include ambient temperature and relative humidity. Most chillers and compressors operate in non-conditioned spaces similar to central chiller plants and mechanical equipment rooms. These spaces are rarely cooled and the humidity within these spaces is rarely controlled. The confluence of these two influences—media temperature and environmental conditions—presents certain challenges. The heat output of the compressor inverter increases as the load of the motor increases. As such, the cooling fluid provided to the inverter needs to be regulated based on the heat load output of the motor because the fluid cooling media is subject to temperature fluctuations. In particular, the amount of cooling fluid must be regulated to the amount of heat that must be rejected.
It is this tight control issue that has stymied many manufacturers and presented problems with safety and limit controls that need to be employed in order to protect the equipment during less than advantageous operating conditions. Many of these safety and limit measures are naturally designed to protect the equipment from operating problems or losses in operation altogether.
In order to maximize the prime function of the safety and protection circuits, the chiller and compressor manufacturers have purposely designed highly reliable shut down and limiting features into the protection systems. The problem with these systems, however, is that they are inherently inefficient systems. They are designed to operate the chiller and compressor motor inverters over a wide range of fluid and temperature conditions. In addition, they limit and/or prevent operation of the motors at higher performance conditions if those conditions create a situation where the inverter could potentially be damaged or fail due to the influence of the outside variables.
For example, the cold condenser water would be limited or prevented from entering the inverter when the environment in the chiller plant is humid. This could create condensation, which could potentially damage the inverter or create a “short circuit” or grounding problem. To avert these problems, chillers or compressors include dew point safety sensors that sense harmful levels of condensation and safety limit devices that reduce the amount of cooling water supplied to the inverter if the dew point safety sensors sense harmful levels of condensation. As a result, the inverter would heat up and the inverter's heat sensors would cause the motor output to slow down or stop altogether.
These safety and limiting features protect the motor inverter, but they also limit the chiller output capacity at a critical point in the chiller efficiency operating curve. This means that the compressor cannot continue to operate or operate at higher capacity at the most opportune operating condition for the chiller. These safety and limiting features severely impede the compressor from operating over a greater span of water, refrigerant, and ambient temperatures. As a result, the compressors are forced to operate at reduced capacities under many conditions and there is an overall reduction in potential kW/ton output over a broad spectrum of environmental conditions. The safety and limiting features also severely limit a chiller's ability to enter into and out of free cooling in a seamless manner. Thus, the safety and limiting features prevent the overall system from functioning seamlessly and at a point when it could be the most beneficial from an energy standpoint.
Compressor manufacturers rate their compressors based on the minimum work load with which they should safely be operated. This is to protect the equipment. This rating is the “lift” of a compressor. The industry standard is currently maintained at approximately 1.5 or above. The rating is a calculation that utilizes the following important information. A set point or standard has been established to maintain a minimum relation between the high and the low system pressure. Because inverters are capable of lowering the speed of the compressor thereby enabling lower operating ranges for the equipment, an industry minimum was established to effectively protect the inverter from operational problems at the low end of the operating parameter. Specifically, the low end of the system is usually related to free cooling and low condenser water conditions. This was mainly due to a concern for condensate and the resultant damage that could occur. As a result of this minimum lift condition to prevent condensation problems, the 1.5 lift has been established as the industry low end safe number.